1. Field of the Invention
The present invention relates generally to light emitting devices and more particularly to light emitting devices using phosphors.
2. Description of Related Art
Blue light emitting diodes (LEDs) are used with luminescent materials (phosphors) to produce light emitting devices which emit apparently white light. U.S. Pat. Nos. 5,813,753 and 5,998,925, for example, disclose light emitting devices in which a blue LED is disposed in a reflective cup and surrounded by material including phosphors. FIG. 1 illustrates such a device. A portion of the blue light emitted by the LED is absorbed by the phosphors, which in response emit red and green light. The combination of the unabsorbed blue light emitted by the LED and the red and the green light emitted by the phosphors can appear white to the human eye.
Typically, light emitted from sources such as the device illustrated in FIG. 1 does not have a uniform color. Often, white light is emitted in a central cone surrounded by annular rings of yellow and blue light. This effect occurs because of a non-uniformity in the thickness of the phosphor-containing material surrounding the LED and consequent spatially non-uniform absorption of blue light and emission of red and green light. In particular, thick regions of phosphor containing material absorb more blue light and emit more red and green light than do thin regions of phosphor containing material. The light from thick regions thus tends to appear yellow, and the light from the thin regions tends to appear blue. As illustrated in FIG. 1, light emitted in path b travels much further through the phosphor than light emitted in path a. When light strikes a phosphor particle, the light is either absorbed and re-emitted at a different wavelength or scattered by the phosphor. Light that travels a longer distance through the phosphor is more likely to be absorbed and re-emitted. Conversely, light that travels a shorter distance through the phosphor is more likely to be scattered out of the device without being absorbed and re-emitted. As a result, more blue light is emitted from regions of the device corresponding to short path lengths through the phosphor and more red and green is emitted from regions of the device corresponding to long path lengths through the phosphor.
U.S. Pat. No. 5,959,316 to Lowery, incorporated herein by reference, proposes eliminating the non-uniformity in path length through the phosphor by depositing a transparent spacer over and around the LED prior to deposition of a uniform thickness of phosphor containing material, as illustrated in FIG. 2. However, surface tension makes the shape and thickness of the phosphor containing material, typically deposited as a liquid or paste (solids dispersed in a liquid), difficult to control. In addition, phosphor layer 66 must be separated from light emitting structure 60. As a result, the effective source size of the light, i.e. the light emitting structure and the phosphor layer, is much larger than the size of the light emitting structure itself. Since the optics used to control the light emitted from the source can grow geometrically with the source size, the large source size proposed by Lowery can present implementation difficulties.
In accordance with one embodiment of the invention, a light source includes a light emitting device, such as a III-nitride light emitting diode, uniformly covered with a luminescent material structure, such as a single layer or multiple layers of phosphor. Any variations in the thickness of the luminescent material structure are less than or equal to 10% of the average thickness of the luminescent material structure. In some embodiments, the thickness of the luminescent material structure is less than 10% of a cross-sectional dimension of the light emitting device. In some embodiments, the luminescent material structure is the only luminescent material through which light emitted from the light emitting device passes. In some embodiments, the luminescent material structure is between about 15 and about 100 microns thick, preferably 15-35 microns thick. In some embodiments, the luminescent material structure includes multiple types of luminescent materials. In some embodiments, the luminescent material structure includes multiple thin layers of luminescent materials.